# Speed limit c in the multiverse

• I
• Spinnor
In summary, the speed of light is essentially a unit conversion within an individual universe, determined by the values of geometric and other moduli in that ground state. The fine structure constant is more fundamental than the speed of light, as it determines the size and lifetime of particles. Velocities in these units are dimensionless, with c = 1.

#### Spinnor

Gold Member
Do we know enough of the workings of string theory to say what factors give rise to a large or small value of the velocity of propagation of massless fields for a given multiverse?

Thanks!

• Limitless101
You can see a zillion threads here asking why the speed of light is what it is, but it is essentially a unit conversion. This question is tantamount to asking if a given multiverse has eleven inches to the foot.

• nikkkom
I guess the speed of light has a definition within an individual universe, with respect to whatever "rods and clocks" (as in special relativity) exist there. Bound systems like atoms will define a natural length scale, and e.g. decay processes will define a natural time scale. So if the question is, what determines those scales for a specific string vacuum, essentially it's the value of the geometric and other moduli in that ground state - size of the extra dimensions, volume of any branes, and so on. These geometric quantities are dynamical and have a lowest-energy configuration, they determine masses and couplings of strings, and masses and couplings in turn determine the size and lifetime of particles.

mitchell porter said:
I guess the speed of light has a definition within an individual universe, with respect to whatever "rods and clocks" (as in special relativity) exist there.
The speed of light is derivative because it is has units. What's fundamental is the fine structure constant. If that's different then the speed of massless objects is different

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You can see a zillion threads here asking why the speed of light is what it is, but it is essentially a unit conversion.